1. Field of the Invention
The present invention relates to an optical transceiver that transmits and receives optical signals.
2. Related Prior Arts
One optical communication system, whose distance between the nearest terminal is shorter than several kilo-meter and transmission speed is relatively slow, below several hundreds of mega-bps, often uses multimode optical fibers and a light-emitting diode (LED) as optical sources. On the other hand, another type of the optical communication system with a long distance and a high transmission speed uses single mode fibers and a laser diode as optical sources.
The FDDI standard, which is one of international standards of the optical communication and rules a distance of 2 km and a transmission speed of 125 Mbps, has adopted the multimode fiber with a core diameter of 62.5 μm and a graded index profile. Another standard of the optical communication system, such as the SDH (Synchronous Digital Hierarchy) whose transmission speeds are 622 Mbps, 2.5 Gbps, 10 Gbps and so on, or the optical Ethernet with transmission speed faster than 1 Gbps, has adopted the single mode fiber and the laser diode (LD) as the optical source.
Recently, it has been requested that existing multimode fibers could be applied to the high speed optical communication system using the LD as the signal source. Because of its larger core diameter, the multimode fiber is easily coupled with the optical source, and consequently, some misalignment between the core and the light source does not result in the larger coupling loss, which makes it easy to produce an optical connector and reduces the production cost. On the other hand, the multimode fiber easily distorts the optical signal transmitted therein because the fiber may transmit a plurality of optical modes and respective modes show different transmission speed within the fiber, which is called as the dispersion. Thus, it has been unable for the multimode fiber to transmit a high speed optical signal.
In order to compensate the dispersion within the multimode fiber, an electrical equalizing process carried out in the optical receiver is known in the field. H. Bulow, et. al has presented this electrical equalizing process in the Optical Fiber Conference in 2002. The electrical equalizing process may be carried out by one type of digital filters with a plurality of delay blocks, a plurality of multipliers, an adder and some control units. The electrical equalizing may realize the high speed transmission over 10 Gbps.
The U.S. Pat. No. 7,147,387 has disclosed another technique in addition to the electrical equalizing in the receiver, where the transmitter compensates the dispersion occurred within the optical fiber, which is called as pre-emphasis. K. Roberts, et al has reported another technique in IEEE Photonics Technology Letters, volume 12 (2), pages 403-405 (2006), that, estimating the dispersion occurred within the optical fiber, the transmitter outputs a pre-distorted optical signal based on the estimation so as to compensate the dispersion of the optical fiber, which is called as the pre-compensation.
In the high speed transmission over 10 Gbps, the frequency bandwidth of the electrical signal lines in the transmitter and the receiver and that of the LD and the PD become a great factor to distort the signal waveform in addition to the dispersion of the optical fiber regardless of single mode or multimode. The electrical equalizing provided only in the receiver sometimes lacks its ability to compensate the distortion due to a limited frequency bandwidth of the electrical signal lines and that of the semiconductor devices. The semiconductor active devices with lower cost thereof that are positively applied in the optical system using the multimode fiber tend to show less bandwidth. Although the prior US patent mentioned above has disclosed the pre-emphasis technique where the optical output from the transmitter is pre-distorted so as to compensate the distortion and the dispersion of the transmission line as monitoring the output waveform from the transmitter, they have not described nor mentioned the relation between the pre-emphasized transmitter and the receiver, and between the pre-emphasized transmitter and the optical fiber.
Moreover, the pre-compensation technique reported by Roberts, et. al is necessary to compensate whole distortion and dispersion due to the signal line, the active devices and the optical fiber only by the pre-compensation, which inevitably requires a large dynamic range of the pre-compensation. A large power is necessary in the calculation to get optimal conditions, which inevitably brings a hard thermal condition for the devices installed within the transceiver.